1. Field of the Invention
The present invention relates to a light emitting diode (LED) and a method of fabricating the same, and more particularly, to an LED to which a substrate separating process is applied, and a method of fabricating the same.
2. Discussion of the Background
A light emitting diode (LED), which is a semiconductor device having a structure in which an N-type semiconductor and a P-type semiconductor are joined together, emits light through the recombination of electrons and holes. LEDs have been widely used as display devices and backlights. Further, LEDs have lower electric power consumption and a longer lifespan as compared with conventional light bulbs or fluorescent lamps, so that their application areas have been expanded to the use for general illumination while substituting for conventional incandescent bulbs and fluorescent lamps.
Recently, alternating current (AC) LEDs that continuously emit light by being directly connected to an AC power source have been commercialized. For example, an LED that may be directly connected to a high-voltage AC power source is disclosed in U.S. Pat. No. 7,417,259, issued to Sakai, et al.
According to Sakai, et al., LED elements (i.e., light emitting cells) are two-dimensionally connected in series on a single insulating substrate such as a sapphire substrate to form LED arrays. Such LED arrays are connected in reverse parallel to one another on the sapphire substrate. As a result, there is provided a single-chip LED which may be driven by an AC power supply.
In the AC LED as described above, light emitting cells are formed on a substrate used as a growth substrate, e.g., a sapphire substrate. Therefore, the structure of the light emitting cells may be restricted, so that there may be a limitation in improving the light extraction efficiency. In order to solve such a problem, a method of fabricating an AC LED to which a substrate separating process is applied has been disclosed in U.S. Application Publication No. 2009/0166645, applied for by Lee.
FIGS. 1 to 4 are sectional views illustrating a method of fabricating an LED according to a related art.
Referring to FIG. 1, semiconductor layers including a buffer layer 23, an N-type semiconductor layer 25, an active layer 27 and a P-type semiconductor layer 29 are formed on a sacrificial substrate 21. A first metal layer 31 is formed on the semiconductor layers, and a second metal layer 53 is formed on a substrate 51 which is discrete from the sacrificial substrate 21. The first metal layer 31 may include a reflective metal layer. The second metal layer 53 is joined with the first metal layer 31 so that the substrate 51 is bonded on the semiconductor layers 25, 27, and 29.
Referring to FIG. 2, after the substrate 51 is bonded, the sacrificial substrate 21 is separated using a laser lift-off process. After the sacrificial substrate 21 is separated, the remaining buffer layer 23 is removed so that a surface of the N-type semiconductor layer 25 is exposed.
Referring to FIG. 3, a photolithography and etching technique is used to pattern the semiconductor layers 25, 27, and 29 and the metal layers 31 and 53, so that metal patterns 40 spaced apart from each other and light emitting cells 30 positioned on partial regions of the respective metal patterns are formed. Each of the light emitting cells 30 includes a patterned P-type semiconductor layer 29a, a patterned active layer 27a and a patterned N-type semiconductor layer 25a. 
Referring to FIG. 4, metal wires 57 are formed to electrically connect upper surfaces of the light emitting cells 30 to the metal patterns 40 adjacent to the light emitting cells 30, respectively. The metal wires 57 connect the light emitting cells 30 to each other, thereby forming a serial array of light emitting cells 30. In order to be connected with the metal wire 57, an electrode pad 55 may be formed on the N-type semiconductor layer 25a, and another electrode pad may be formed on the metal pattern 40. Two or more arrays may be formed, and these arrays are connected in reverse parallel, thereby providing an LED capable of being driven under AC power.
According to the related art as described above, the material comprising the substrate 51 may be variously selected to improve the heat dissipation performance of the LED, and a surface of the N-type semiconductor layer 25a may be treated to enhance the light extraction efficiency of the LED. Further, since a first metal layer 31a includes a reflective metal layer to reflect the light which runs from the light emitting cells 30 toward the substrate 51, the light emitting efficiency may be further enhanced.
However, while the semiconductor layers 25, 27 and 29 and the metal layers 31 and 53 in the related art are patterned, etching by-products of metal materials may be stuck to sidewalls of the light emitting cell 30, so that an electrical short circuit may be caused between the N-type and P-type semiconductor layers 25a and 29a. Further, a surface of the first metal layer 31a exposed during the etching of the semiconductor layers 25, 27 and 29 may be easily damaged by plasma. If the first metal layer 31a includes a reflective metal layer such as Ag or Al, such etching damage may increase, causing the LED to deteriorate. The damage of the surface of the metal layer 31a caused by plasma may decrease the adhesion of the wires 57 or the electrode pads 55 which are formed on the surface thereof, and may thereby result in a device failure.
Meanwhile, according to the related art, since the first metal layer 31a may include a reflective metal layer, light emitted from the active layers 27a toward the substrate 51 from the light emitting cells 30 may be reflected away from the substrate 51. However, light may not be reflected in spaces between the light emitting cells 30 due to etching damage or oxidation of the reflective metal layer. Further, because the reflective metal layer may have a maximum reflectance of about 90%, there may be a limitation in improving the reflectance. Furthermore, since the substrate 51 is exposed in regions between the metal patterns 40, light may be absorbed by the substrate 51.
In addition, since the wires 57 are connected onto upper surfaces, i.e., light emitting surfaces, of the N-type semiconductor layers 25a, respectively, the light generated in the active layers 27a may be absorbed by the wires 57 and/or the electrode pads 55 on the light emitting surfaces, so that light loss may occur.
FIG. 5 is a sectional view illustrating an LED having light emitting cells connected in series according to the related art.
Referring to FIG. 5, the LED includes a substrate 51, a bonding metal 41, an adhesive layer 39, an intermediate insulating layer 37, a barrier metal layer 35, a reflective metal layer 33, light emitting cells S1 and S2, an insulating layer 63, and a connector 65.
The substrate 51 is distinguished from a growth substrate (not shown), and is a secondary substrate bonded to nitride semiconductor layers 25, 27, and 29 through the bonding metal 41 after the nitride semiconductor layers 25, 27, and 29 are grown on the growth substrate.
Meanwhile, each of the light emitting cells S1 and S2 includes an n-type nitride semiconductor layer 25, an active layer 27, and a p-type nitride semiconductor layer 29, and an upper surface of the n-type nitride semiconductor layer 25 may be configured to be a roughened surface R.
The intermediate insulating layer 37 is interposed between the substrate 51 and the light emitting cells S1 and S2 so that the light emitting cells S1 and S2 are electrically insulated from the substrate 51. The reflective metal layer 33 and the barrier metal layer 35 are also interposed between the intermediate insulating layer 37 and the light emitting cells S1 and S2. The reflective metal layer 33 reflects light which is generated in the light emitting cells S1 and S2 and is emitted towards the substrate 51, thereby improving the light emitting efficiency. The barrier metal layer 35 covers the reflective metal layer 33 so that the barrier metal layer 35 may prevent the diffusion of the reflective metal layer 33 and the oxidation of the reflective metal layer 33. Further, a portion of the barrier metal layer 35 is exposed by being extended from a region below the light emitting cell S2 to a cell separation region.
The connector 65 connects the n-type semiconductor layer 25 of the light emitting cell S1 to the barrier metal layer 35 so that the light emitting cells S1 and S2 are connected in series. The insulating layer 63 is interposed between the connector 65 and the light emitting cells S1 and S2 to prevent the n-type and p-type semiconductor layers 25 and 29 from being electrically short-circuited by the connector 65.
Silver (Ag) may be used as the reflective metal layer 33. Ag may be easily oxidized and diffused by heat. Further, an etching gas, e.g., BCl3/Cl2 gas, used to separate the light emitting cells S1 and S2 may easily produce etching by-products through chemical reaction with the Ag. The etching by-products may be stuck to side surfaces of the light emitting cells S1 and S2, and therefore, an electric short circuit may be caused. In order to prevent the electric short circuit in the related art, the reflective metal layer 33 is covered with the barrier metal layer 35, and then the barrier metal layer 35 is configured to be exposed in a separation process of the light emitting cells S1 and S2.
However, since the barrier metal layer 35 is added to protect the reflective metal layer 33 according to the related art, a metal layer deposition process may be complicated. Further, since the reflective metal layer 33 is formed and the barrier metal layer 35 then covers the reflective metal layer 33, a step occurs in a side surface of the reflective metal layer 33. The step increases as the thickness of the reflective metal layer 33 increases. Particularly, if a plurality of metal layers is deposited to form the barrier metal layer 35, stresses may be concentrated around the step, so that cracks may be produced in the barrier metal layer 35. Particularly, since the substrate 51 is bonded at a relatively high temperature, cracks may be produced at the step during the bonding of the substrate 51, so that a device failure may be caused.
Meanwhile, if the reflective metal layer 33 is formed of Ag, the reflectance of the reflective metal layer 33 may be increased compared with forming the reflective metal layer 33 of other metals.